The present invention relates to a control apparatus for a drilling machine and, more particularly, to a portable control apparatus for a drilling machine which includes an electric drill having an electromagnetic base (electromagnet unit) at its lower portion for fixing the electric drill on a workpiece and a drilling tool, e.g., an annular drilling tool; and a motor (to be referred to as a feed motor hereinafter) for feed-control of the electric drill.
As is well known, electric drills are classified into two types, i.e., one using an annular drilling tool and the other using a twist drill. An electric drill having an annular drilling tool can provide a larger torque and has a higher cutting capacity than the one having a twist drill. On the other hand, an abnormally large load may act on the annular drilling tool, so that the annular drilling tool is sometimes damaged or the electric drill is burned.
Therefore, when an operator operates the electric drill with the annular drilling tool, he must always pay careful attention so as not to cause a large load to act on the annular drilling tool.
However, only an extremely skilled operator can recognize the magnitude of the load, i.e., determine whether a proper load acts on an edge of the annular drilling tool or too large or too small a load acts thereon.
When an unskilled operator operates a drilling machine, the annular drilling tool may be damaged or the electric drill may be burned as described above. In an extreme case, the electric drill itself may be rotated about the distal end of the annular drilling tool.
In addition, a power source of the drilling machine is preferably turned off immediately after completion of drilling to save power consumption and to prevent damage to the annular drilling tool. However, according to a conventional drilling machine, an operator must recognize completion of drilling and then turn off a power source switch, resulting in a cumbersome operation for an operator and poor operability.
In order to eliminate the above drawbacks, the present inventors have made various researches and developments, and have already filed Japanese Patent Application Nos. 56-186473, 57-23741, and 59-252697. The U.S. Patent Application corresponding to Japanese Patent Application Nos. 56-186473 and 57-23741 was issued on Aug. 5, 1986 as U.S. Pat. No. 4,604,006.
An invention disclosed in U.S. Pat. No. 4,604,006 includes a feed motor for moving an electric drill toward a workpiece, and a detector is provided to a control circuit of the electric drill to detect the magnitude of a load of the electric drill on the basis of a current flowing therethrough. When a detection output reaches a first preset level, the detector auomatically stops feed of the electric drill to decrease the load. When the detector detects a decrease in the load, it starts feed of the electric drill again, thereby intermittently driving the feed motor. In addition, when the detection output reaches a second preset level, it automatically stops both rotation and feed of the electric drill. When the detector detects that drilling is completed, it also automatically stops both rotation and feed of the electric drill.
An invention disclosed in Japanese Patent Application No. 59-252697 includes a first detector for stopping rotation of a feed motor when a load current flowing through a drill motor exceeds a first threshold value and a second detector for stopping rotation of the drill motor when the load current exceeds a second threshold value, and further includes a controller for intermittently stopping the rotation of the feed motor when the load current is less than the first threshold value. As a result, removal of a built-up edge formed on the tool and discharge of chips can be easily performed, thereby preventing a large load from abruptly acting on the electric drill.
However, the above conventional techniques have the following problems.
(1) When a feed motor is intermittently driven, drilling efficiency is decreased.
(2) Rotation speed of a feed motor and hence feed speed of an electric drill are conventionally set at substantially constant values. Especially when a workpiece has a high-hardness layer called a "mill scale" on its surface, penetration of a drill is poor in an initial drilling period. As a result, drilling positioning may be inaccurate or offset, or in an extreme case, a drilling machine itself may be rotated.